The present invention relates generally to use of induction type energy harvesters along with DC-DC converters, and more particularly to switching circuitry and techniques for using the inductance of the energy harvester both for inductive energy harvesting and for performing DC-DC conversion of the energy harvester output.
The basic structure of an induction type vibration energy harvester 1 is shown in FIG. 1. It includes a mass m and associated coil or inductor 4 suspended on one end of a spring 2. The mass m and coil 4 move with velocity v relative to a magnet 5. The other end of spring 2 and magnet 5 are supported by a support 6. (The coil might be stationary with the magnet moving, or vice versa.). When coil 4 and associated mass m move with velocity v in the magnetic field, their kinetic energy Ek=(mv2)/2 is transformed into potential energy in spring 2 which then is converted into electromagnetic energy ELh=eL=(LhILh2)/2. The electromotive force eL in the coil is defined by the velocity v of its movement through the magnetic field, and is given by eL=−w×B×v, where w is number of turns of the coil, B is the magnetic field density, and v is the velocity of coil 4 relative to magnet 5. The frequency of this AC current is the frequency of vibration, typically 60 to 120 Hz, and, at the present state-of-the-art, no more than approximately 2000 Hz. More generally, the maximum frequency of the AC current to be harvested will be no more than about 10 times the switching frequency of the switches being utilized in the power management circuitry.
A known power management circuit 10 including an induction-type energy harvester 1 is shown in FIG. 2. Power management circuit 10 also includes a rectifier and a low dropout (LDO) regulator. A passive rectifier including diodes S1, S2, S3, and S4 rectifies the AC output signal produced between terminals 7A and 7B by inductive energy harvester 1. A cathode of diode S1 is connected to VDD and its anode is connected by terminal 7A to the cathode of diode S3, the anode of which is connected to ground. The cathode of diode S2 is connected to conductor 18. The anode of diode S2 is connected by terminal 7B to the cathode of diode S4, the anode of which is connected to ground. A filter capacitor C0 is connected between VDD and ground. A DC-DC converter, such as LDO regulator 11, is connected between VDD and ground to produce a regulated output voltage VCAP on output conductor 12. A load capacitor CL is connected between output conductor 12 and ground. LDO 11 includes a P-channel transistor M1 connected between VDD and output conductor 12. A resistor R is connected between VDD and the cathode of a zener diode D, the anode of which is connected to ground. The junction between resistor R and zener diode D is connected to the gate of transistor M1.
The output impedance of induction energy harvester 1 and the input capacitance of the energy storage circuitry forms a resistor-capacitor network that typically has a time constant of roughly 20 seconds which determines how quickly the voltage on the storage capacitor CL rises. Consequently, at low vibration levels it may take several minutes before there is sufficient energy to acquire a vibration spectra and transmit it. To avoid this problem, resistor R2, zener diode D5 and transistor M1 all of LDO regulator 11 in FIG. 2 isolate the large storage capacitor CL from C0. This allows the voltage VDD to establish itself much faster, e.g., 30 times faster.
Induction harvester power management system 10 of FIG. 2 has the shortcoming that its harvesting efficiency is low due to losses in the rectifier diodes. Another shortcoming is that it does not allow any energy at all to be harvested at very low vibration levels (for example, when VDD<VCAP) because it does not develop enough voltage to adequately charge filter capacitor C0.
The efficiency of power management system 10 in Prior Art FIG. 2 can be improved by using a DC-DC converter including an inductor, and low-vibration-level harvesting capability can be improved by choosing a DC-DC converter of the boost type as shown in Prior Art FIGS. 3A and 3B.
In FIG. 3A, prior art power management circuit 15-1 includes an inductive energy harvester 1, for example as shown in FIG. 1. A passive rectifier including diodes S1, S2, S3, and S4 rectifies the AC output signal produced by inductive energy harvester 1 between its terminals 7A and 7B. A cathode of diode S1 is connected to conductor 18, and its anode is connected by conductor 7A to the cathode of diode S2, the anode of which is connected to ground. The cathode of diode S3 is connected to conductor 18. The anode of diode S3 is connected by conductor 7B to the cathode of diode S4, the anode of which is connected to ground. A filter capacitor C0 is connected between conductor 18 and ground.
Power management circuit 15-1 also includes a boost converter circuit including inductor L1, a switch S5, a diode S6, and a switch control circuit 17. A first terminal of inductor L1 is connected to receive a rectified voltage VL1 produced on conductor 18 by energy harvester 1 and the passive rectifier S1,2,3,4. VL1 is also applied to an input of switch control circuit 17. The other terminal of inductor L1 is connected by conductor 16 to one terminal of switch S5 and to the anode of diode S6. The other terminal of switch S5 is connected to ground. The control electrode of switch S5 is connected by conductor 14 to an output of switch control circuit 17.
FIG. 3B shows a more detailed implementation 15-2 of power management system 15-1 of FIG. 3A. In FIG. 3B, prior art power management system 15-2 includes inductive or vibration harvester 1 and passive rectifier S1, 2 ,3, 4 connected as in FIG. 3A. Switch control circuit 17 in FIG. 3B includes a comparator A0 having its inverting input coupled to receive a threshold voltage Vhrv and its non-inverting input connected to receive VL1 on conductor 18 such that boosting operation of the DC-DC boost converter circuitry occurs when VL1 exceeds Vhrv. The output of comparator A0 is connected to an input of a logic circuit 13. Another input of logic circuit 13 receives a “battery charged” or “battery full” input signal supplied by conventional voltage sensing circuitry. In FIG. 3B, the lower terminal of switch S5 is coupled to one terminal of a current sensor 113 a terminal of which is connected to ground. An output of current sensor 113 is connected to an inverting input of a comparator A1 which receives a reference threshold value Imax on its non-inverting input. The output of comparator A1 is connected to another input of logic circuit 13. The output of logic circuit 13 is connected by conductor 14 to the control terminal of switch S5. Diode S6 as shown in FIG. 3A is implemented in FIG. 3B by means of a synchronous rectifier which includes switch S6 coupled between output conductor 19 and conductor 16, with its control terminal coupled to the output of a comparator A2 having its inverting input connected to output conductor 19 and its non-inverting input connected to inductor terminal 16.
The power management system of Prior Art FIGS. 3A and 3B replaces the LDO regulator in prior art FIG. 2 with a boost converter. The power management systems 15-1 and 15-2 of FIGS. 3A and 3B require an inductor (L1), which typically would be a large, expensive, external component. The system of FIGS. 3A and 3B also has the shortcoming of excessive voltage and power loss in the 4 diodes of the passive rectifier S1, 2, 3, 4, and (3). Furthermore, another DC-DC converter or LDO regulator would be required between the battery and load in order to provide a regulated supply voltage.
The relevant prior art is believed to also include commonly assigned U.S. Pat. No. 6,275,016 entitled “Buck-Boost Switching Regulator” issued Aug. 14, 2001 to Vadim V. Ivanov and incorporated herein by reference.
Thus, there is an unmet need for a way to avoid the high cost of the coil or inductor usually required in a DC-DC converter utilized for conversion of the AC output generated by an induction energy harvester.
There also is an unmet need for an inductive energy harvesting device which is more efficient than those available in the prior art and which avoids voltage losses in rectifier diodes.
There also is an unmet need for an inductive energy harvesting device which is more efficient than those available in the prior art and which is capable of efficient harvesting of very low AC voltage levels produced by relative movement between a coil and a magnet thereof in a low-vibration-level environment.
There also is an unmet need for an inductive energy harvesting device which is more efficient and effective than those available in the prior art and which avoids voltage losses in rectifier diodes in prior art inductive energy harvesting devices and which is capable of more efficient harvesting of very low AC voltage levels produced by relative movement between a coil and a magnet thereof in a low-vibration-level environment than has been achievable in the prior art.